The video shows accused killer Aaron Alexis stalking the hallways of building 197 with a shotgun.
Investigators say he killed 12 people and wounded four more before he was killed by police.
In the nine days since the attack there have been reports Alexis had struggled for years with psychological problems and complained of hearing "voices."
"Alexis held a delusional belief that he was being controlled or influenced by extremely low frequency or ELF, electro magnetic waves," said FBI Assistant Director in Charge Valerie Parlave.
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